1. Field of the Invention
The present invention relates to an optical pickup, an optical element used in same, and an information reproducing apparatus including same.
2. Related Art
There is known in the related art a compatible information reproducing (recording) apparatus that can reproduce (record) optical discs with different standards such as CD, DVD. This compatible information reproducing (recording) apparatus need store produce (record) the optical disc for each standard by focusing a laser beam with a specified wavelength to a required spot diameter on the recording surface of the optical disc.
The protective layer on the optical disc may have different thickness for each standard. For the protective layer with different thickness, the movement of the objective lens to adjust the focal point of the laser beam is insufficient to remove the spherical aberration and is incapable of focusing the laser beam to the required spot diameter, making it difficult to reproduce (record) the information.
To correct this spherical aberration, the conventional compatible information reproducing (recording) apparatus includes a diffractive lens structure formed over at least the whole of an area of a smaller effective diameter of effective diameters of the objective lens included in the optical pickup which are necessary for the reproduction of plural types of information recording media (see, for example, Japanese application patent laid-open publication No. 2000-81566).
The related art as described above has following problems.
The die material generally has a higher thermal expansion coefficient than the lens material. The die will thus contract more than the lens when they are cooled after molding, thereby making it difficult to release the lens. The lens made from resin may cause destruction of the lens surface fine structures or cause deformation of the lens when it is released, thereby increasing the aberration. The lens made from glass may destroy the die.
The diffractive structure provided on the lens has narrower distances between the phase steps at the outer periphery of the lens. This may decrease the diffraction efficiency at the lens periphery due to the edge shadow of the steps. Particularly, in the inner common area in which the diffractive lens structure is provided, the slight variation of the wavelength used may vary the aberration as well as the image point position. The outer area intended for the reproduction of the high-density optical disc thus also needs the diffractive structure to prevent the discontinuous wavefront between the inner area and the outer area. This may decrease significantly the use efficiency of the light beam for the high-density disc.
The problems to be solved by the present invention include the above-described problem by way of an example.